Exhaust emissions from motorized on-highway machines are regulated by the federal government and must not exceed certain contaminant levels as set forth in Title 40, Chapter 1 of the Code of Federal Regulations, Section 86 Subpart C. For example, some government regulations place limits on the amount of particulate matter that may be emitted by diesel truck engines. These regulations specify the acceptable level of particulate matter that may be carried in the exhaust gas stream of the engine. Particulate matter may include, for example, carbon particulates, unburned hydrocarbons, and sulphates. Initial regulations primarily addressed particulate matter levels when the engine was operating in a relatively steady state condition. More recent regulations have been promulgated that regulate the amount of particulate matter when the engine is undergoing a transient condition, such as accelerating from one speed and load condition to another speed and load.
Due to these regulations, equipment has been developed to test and analyze machine engines and/or other power sources for conformance with government standards. In particular, partial flow exhaust gas sampling systems have been developed in an effort to certify such power sources as being in compliance with government emissions regulations. Generally, these systems operate by extracting a small portion of a power source exhaust flow via a test probe located in the exhaust stack. A regulated flow of filtered ambient air is then mixed with the extracted portion, and the combined flow is directed to a filter configured to trap the particulate matter contained within the combined flow. The power source may then be evaluated based on the quantity of particulate matter trapped by the filter during a particular test cycle. One particulate sampling system and method that has performed well in recent years is shown and described in co-owned U.S. Pat. No. 7,299,690. However, with the onset of additional regulations for testing exhaust for particulate matter content during transient conditions, new previously unaddressed problems have arose. For instance, because of certain system parameters including a fixed flow area opening on the test probe as well as other phenomenon, such as the pressure rise in the exhaust stack during accelerating transient conditions, current systems tend to oversample during pressure increases in the exhaust stack which can lead to an inaccurate assessment of particulate matter production during the transient.
Another such system is described in U.S. Pat. No. 6,062,092 to Weaver (“the '092 patent). The system of the '092 patent utilizes a feedback arrangement to change the proportion of exhaust gas being extracted from the power source exhaust flow relative to the total flow of exhaust. Although the system changes this proportion based on changes in the exhaust flow of the power source, the system uses the pressure differential between the exhaust gas flow stream and the pressure inside a sampling probe for feedback. As a result, the system of the '092 patent does not take into account variations in the combined flow seen by the filter when determining the amount of exhaust flow to extract, which can again lead to oversampling and inaccurate results. Such variations may be caused by, for example, exhaust flow excursions and/or other system based factors.
The systems and methods of the present disclosure are directed to overcome one or more of the problems set forth above.